The variable itself isn't in a list context; whatever comes after the equal sign is in a list context. Regular expression matches are one place where this matters, but here's a clearer example:
$ perl -le 'my $x = localtime; print $x'
Tue Apr 4 11:25:49 2006
$ perl -le 'my($x) = localtime; print $x'
52
If you read the docs for localtime, you'll see that it returns different values when called in a scalar or list context, so the type of assignment can make a big difference.
Here's another example:
#!/usr/bin/perl
sub test
{
if (wantarray) { return ('list context'); }
else { return 'scalar context'; }
}
my $s = test;
print "s=$s\n";
my(@l) = test;
print "l=@l\n";
which outputs:
s=scalar context
l=list context